// 有一分数序列： 2/1 3/2 5/3 8/5 13/8 21/13...... 求出这个数列的前N项之和，保留两位小数。
#include<stdio.h>
int main(){
    double N, numerator, denominator,Sum=0,a=1.0, b=1.0;
    scanf("%lf", &N);
    for (int i=1; i<=N;i++){
        numerator= a + b; //3 5 8
        denominator = a ;  //2 3
        Sum+=numerator/denominator;
        a = numerator; //3 5
        b = denominator; //2 3
        // printf("%lf %lf %lf %lf %lf \n", Sum, numerator, denominator, a, b);
    }
    printf("%0.2lf", Sum);
}